the price difference is about £30-40 and I don't really see this replacing usb unless peripherals using thunderbolt really cut down their prices. Unless of course your a high end user who needs fast speeds for video capture, etc.
Thunderbolt is relatively new on the PC side, but the prices have been going down which is a good sign. Footnote - ASUS has the wrong specs listed on their website and the P8Z77-V PRO/THUNDERBOLT is a 2-WAY SLI/3-WAY CF (listed as Supports NVIDIA® 3-Way SLI™ Technology / Supports AMD Quad-GPU CrossFireX™ Technology) I looked at the manual for indications of a PLX chipset and it lists under 3-WAY CF only and x8/x4/x4 which won't work in SLI.
Thunderbolt peripherals are few and far between, but there's a distinct advantage (bandwidth; 10 Gbit/s -- SATA3 is 6 Gbit/s) if you need a large external or daisy chained external drives or have a Thunderbolt monitor.
Thunderbolt right now is just like SSDs were when they first came out. The price is dropping, but at a very low rate. It's actually useless to buy the V-PRO Thunderbolt right now. You can always get a cheap expansion thunderbolt card or something in the future.